Years ago a coworker told me he used to have problems with mosquitoes too until he began tucking a dryer sheet into his shirt collar.
That is when I started thinking, ya I know, dangerous. mosquitoes find their victims by smell, so if you mask the odour that smells good to skeeters they will...
Maybe it has something to do with the ratio of vanilla smell to chicken poop smell. If the vanilla smell is stronger, than no flies; If the poop smell is stronger, flies.